Perl中defined用来判断一个变量是不是undef，也就是知道有这个变量但不知道它是不是undef；exists用来判断一个hash中的键是否存在。键存在但有可能是undef。

#!/usr/bin/perl -w
use strict;

my $a;
my $b="";
my $c=1;
print "a ok\n" if defined $a;
print "b ok\n" if defined $b;
print "c ok\n" if defined $c;
#print "a exists\n" if exists $a;

my %hash = (
            'aa' => 'aaaa',);
if(exists $hash{'aa'}){
    print "aa exists\n";
}else{
    print "aa doesn't exists\n";
}

my $vv = undef;
my %hash;
$hash{111} = $vv;

print (exists $hash{111} ? "ok\n" : "Oops\n");
print (defined $hash{111} ? "ok\n" : "Oops\n");